Conventionally, there has been provided a programmable controller (PLC) in which an extended I/O unit can be attached (see, e.g., Patent Document 1).
FIG. 7A illustrates an example of a building block type programmable controller including a power unit 1, a CPU unit 2 and a plurality of (six in the example of FIG. 7A) I/O units 3 which are mounted on a backplane 5. In the programmable controller, a system power generated from the power unit 1 is supplied to the CPU unit 2 and each of the I/O units 3 via an internal bus provided in the backplane 5.
Further, FIG. 7B illustrates an example of a stack type programmable controller, which includes a power unit 1, a CPU unit 2 and a plurality of (six in the example of FIG. 7B) I/O units 3. Since the stack type programmable controller does not include the backplane 5 unlike the building block type programmable controller, the units are fixed by connecting one unit to a next unit. A system power generated from the power unit 1 is supplied to the CPU unit 2 and each of the I/O units 3 via stack connectors 6.
[Patent Document 1] Japanese Patent Application Publication No. 2006-79361 (Paragraphs [0014]-[0016] and FIG. 2)-09P00835
[Patent Document 2] Japanese Patent Application Publication No. 2009-147997 (Paragraphs [0016]-[0023] and FIG. 1)-09P00833
[Patent Document 3] Japanese Patent Application Publication No. 2000-105521 (Paragraphs [0057]-[0060] and FIGS. 10 and 11)-09P00834
Each of the above-described conventional programmable controllers includes a back-up power source to perform a termination process (e.g., data back-up or the like) even when a power supply is shut down due to a sudden power failure or the like. However, a back-up time provided from the back-up power source depends on the capacitance of a capacitor thereof and, for example, when a system becomes complicated and processes a large amount of data, the back-up power source may be turned off before completing the termination process.
Further, in the programmable controller shown in FIGS. 7A and 7B, the system power generated from the power unit 1 is simultaneously supplied to the CPU unit 2 and each of the I/O units 3 and the capacity of the power unit 1 may be large enough to satisfy normal consumption power depending on the number of the connected I/O units 3 and specifications thereof. In the start-up, however, the consumption power may exceed the capacity of the power unit 1 and it may be impossible to normally start the system.
In order to solve the above problem, there is proposed a programmable controller in which the I/O units are started sequentially in a specific order (see, e.g., Patent Document 2). In this programmable controller, start-up circuits with different time constants from each other are respectively provided in the I/O units and each I/O unit starts its own power circuit at a start-up timing corresponding to its own time constant.
The programmable controller disclosed in Patent Document 2 is configured such that the respective power circuits of the I/O units have different start-up timings in order to prevent the consumption power in the start-up from exceeding the capacity of the power unit. Accordingly, the system can be normally started. However, the start-up time of each I/O unit depends on a load, circuit configuration and the like and it is difficult to set an optimal start-up time while taking them into account.
Meanwhile, when electric power is supplied to the programmable controller shown in FIGS. 7A and 7B and all I/O units 3 are initialized, the CPU unit 2 identifies each of the I/O units 3 and starts communications with the I/O units 3 to execute a sequence program.
There is disclosed an apparatus in which communications are not started until all extension units are initialized (see, e.g., Patent Document 2). In the apparatus, extension units connected to a main unit are initialized sequentially from the one at the downstream side to the one at the upstream side, and an upstream side extension unit begins to be initialized upon detecting an initialization completion signal from a downstream side extension unit. Further, when the main unit detects an initialization completion signal from the most upstream side extension unit, it determines that all extension units have been initialized and starts communications with each of the extension units.
In the above-described apparatus, communications are not performed between the main unit and the extension units until all extension units are initialized. However, since the upstream side extension unit begins to be initialized upon detecting an initialization completion signal from the downstream side addition unit, the start-up time of an extension unit accumulatively increases. As a result, it may require some time until the system is started after all extension units have been initialized. In this case, the time until the system is started means the time until the CPU unit identifies each extension unit and starts communications with each extension unit after all extension units have been initialized.